A new polypropylene (PP) plant was built by BP Amoco Chemical at Geel, Belgium in 1995-96. This project involved construction of a 200,000 tonnes/year PP unit using the BP Amoco gas phase process and BP Amoco's high-activity supported catalyst. The plant produces random and blocked copolymers, and a wide range of homopolymers. When it came on line in late 1996, it doubled BP Amoco's European PP capacity.

A purge column was included in the design following the reactors. The purge column provides several important functions in a gas phase process. First, it reduces residual volatiles to an acceptable level using nitrogen as the purge gas. Second, it deactivates the trace catalyst residues by using a small quantity of steam. Third, it provides surge capacity between the reaction and pelletization operations.

Gravity flow processing vessels are used for numerous functions within chemical processing plants. One common application is as a purge column, such as those used just after a reactor in a polyethylene or polypropylene facility to reduce volatiles to acceptable levels. Compared to processing vessels which involve mechanical agitation or a fluid bed, gravity flow processing vessels provide numerous benefits, including relatively gentle handling of particles, significant surge capacity while processing takes place, and reasonable cost.

In spite of their benefits, problems can occur with gravity flow processing vessels that limit their effectiveness. Incomplete purging or nonuniform conditioning, caused by localized fluidization, nonuniform gas distribution, nonuniform velocity profiles of the solid particles, and poor handling characteristics of the bulk solid, result in off-grade product, fire/explosion risks, environmental compliance issues, or downstream handling problems. Other problems include excess gas usage and cross-contamination during grade changeovers.

While the given problems are common, they can be prevented or minimized. The first steps are to define the process requirements and to determine the properties of the solids and gas that are being used. The key properties of the bulk solid are those which relate to how it flows in the vessel. These include its cohesive strength, wall friction, range of bulk density, and permeability.

Constraints on the vessel design must also be considered. Parameters such as minimum residence time, maximum storage capacity, vessel diameter, hopper geometry, and gas distribution method(s) are all important. The interrelationships between these parameters are also critical.

Once the material's flow properties are known, one can calculate the type of flow pattern which will occur in a vessel having a particular geometry and size. One of two basic flow patterns may exist: funnel flow and mass flow. The conditions under which each occurs have been well documented in the literature and proven through 40+ years of field experience.

For this application, engineers at BP Amoco specified the minimum and maximum purge duration (i.e., time of exposure to purge gas). They also specified the required solids and gas flow rates as well as the gas temperature and pressure at the top of the vessel. Based on material flow property data generated during previous projects for BP Amoco, engineers at Jenike & Johanson developed the functional design of this purge column. It consists of a tall, narrow cylinder section, below which is a steep-sided conical hopper. Purge gas is introduced just above the top of the conical hopper section using our patented J-Purge™ technology. The design provides a uniform distribution of the purge gas, with no adverse effects on the flowing solids.

The purge column has been in operation now for several years and has performed very well, meeting the design basis. According to Jim Lee, Research Associate for BP Amoco Chemical, “The performance of the purge column has exceeded our expectations. In fact, it did not require any modification when the rest of the PP unit was debottlenecked by 40%.” This technology has been incorporated into three other plants which use BP Amoco's gas phase process: BP Amoco's Chocolate Bayou No. 4 PP, shown in the photo, and two licensees.